Hybrid vehicles and electric vehicles have recently been of great interest as environment-friendly vehicles. The hybrid vehicles are now partially commercialized.
A hybrid vehicle has, as its motive power sources, a DC (direct current) power supply, an inverter and a motor driven by the inverter in addition to a conventional engine. More specifically, the engine is driven to secure the motive power source and a DC voltage from the DC power supply is converted by the inverter into an AC (alternating current) voltage to be used for rotating the motor and thereby securing the motive power source as well. An electric vehicle refers to a vehicle that has, as its motive power sources, a DC power supply, an inverter and a motor driven by the inverter.
Regarding the hybrid and electric vehicles, it has been proposed to boost a DC voltage from the power supply with a voltage step-up converter and invert the boosted DC voltage into an AC voltage for driving the motor.
Japanese Patent Laying-Open No. 2000-333465 discloses a system having a converter variably changing an input voltage to an inverter which drives a motor. According to the input voltage to the inverter and a voltage required for controlling the motor, the control mode of the motor is switched from a pulse-width-modulation control mode (PWM control mode) to a rectangular-wave control mode.
If the motor is driven in the rectangular-wave control mode while a DC voltage from a power supply is boosted and then supplied to the inverter, however, the amount of current taken from the power supply increases, resulting in a problem of generation of overcurrent.